AUTHOR JK Rowling is promising fans of Harry Potter a Hallowe'en treat after she announced plans for a one-off special website story about the wizard.

The Edinburgh-based author has penned the tale which is due to appear on the Harry Potter website in time for the ghostly festival.

It centres on cruel Hogwarts professor Dolores Umbridge, a witch who first appeared in Harry Potter And The Order of Phoenix as the Minister for Magic's Senior Undersecretary. The professor was played by Imelda Staunton in the hit movies.

According to Pottermore.com, which has been enchanting fans by dropping a few hints about the original work, it will be a "rich, 1700-word back story about Umbridge's life, filled with many new details, as well as Rowling's revealing first-person thoughts and reflections about the character".

Rowling released another Harry Potter story on the internet in the summer to coincide with the Brazil World Cup.

The storyline saw 33-year-old, greying Harry reunite with Ron Weasley, Hermione Grainger, Luna Lovegood and Neville Longbottom at the Quidditch World Cup final.

The resulting enthusiasm of Potter fans caused the website to crash.

Professor Umbridge famously ordered Harry to use a Blood Quill and scar himself with the phrase: "I must not tell lies."

A statement on the website said: "Umbridge is not only one of the most malicious Potter characters, she is the only person other than Lord Voldemort to leave a permanent physical scar," reads a statement on Pottermore.com.

Rowling was forced to deny earlier this week reports that she has completed a romantic novel, having branched out into crime writing with two books under the pseudonym Robert Galbraith.

She is alleged to have been out celebrating in London the book's completion, which she denies."I sometimes have a drink even when I haven't finished a book," she wrote on Twitter. "Yes, that's how rock and roll I really am."

Instead, Rowling is said to be making good progress what is believed to be a follow-up to her best-seller The Casual Vacancy, which was her first published foray away from Potter.

The website story is expected to appear on Friday.